Magnetron power supply having in-rush current limiter

ABSTRACT

In a microwave oven which includes a cooking cavity, a magnetron for supplying microwave energy into the cooking cavity, a power supply of the type which supplies power to a magnetron and containing a filament transformer for stepping down line AC voltage to the lower voltage level for the magnetron heater, and an oven lamp for illuminating the oven cavity. The lamp normally is electrically in series with the primary winding of the filament transformer and current through the filament transformer is thereby limited resulting in minimal heater current to the magnetron heater. A relay or other controlling switch is energized by operation of an oven cavity door switch upon the first opening of the oven cavity door. The relay operates, interrupts the described series circuit, and places both the primary winding of the filament transformer and the lamp in parallel each across the AC supply lines so that full voltage is thereafter applied to both the lamp and filament transformer.

Jan. 9, 1973 [54] MAGNETRON POWER SUPPLY HAVING IN-RUSH CURRENT LIMITERPaul Wythe Crapuchettes, Woodside, Calif.

Litton Systems, Inc., San Carlos, Calif.

Filed: Dec 6, 1971 Appl. No.: 204,851

Inventor:

1 References Cited UNITED STATES PATENTS L emmon ..328/270 PrimaryExaminer-J. V. Truhe Assistant Examinerl-1ugh D. Jaeger Attorney-RonaldM. Goldman et al.

[ ABSTRACT In a microwave oven which includes a cooking cavity, amagnetron for supplying microwave energy into the cooking cavity, apower supply of the type which supplies power to a magnetron andcontaining a filament transformer for Stepping down line AC voltage tothe lower voltage level for the magnetron heater, and an oven lamp forilluminating the oven cavity. The lamp normally is electrically inseries with the primary winding of the filament transformer and currentthrough the filament transformer is thereby limited resulting in minimalheater current to the magnetron heater. A relay or other controllingswitch is energized by operation of an oven cavity door switch upon thefirst opening of the oven cavity door. The relay operates, interruptsthe described series circuit, and places both the primary winding of thefilament transformer and the lamp in parallel each across the AC supplylines so that full voltage is thereafter applied to both the lamp andfilament transformer.

8 Claims, 2 Drawing Figures It m b3 1 2/ SUPPLY L 591 3 36 E l b, 1-----1 3/ TIMER AND OTHER 4/ CONTROL 1 37 1 E'Z B'I J FIELD OF THEINVENTION This invention relates to microwave ovens and, moreparticularly, to microwave oven power supplies containing oven lamps andfilament transformers.

BACKGROUND OF THE INVENTION Microwave ovens include a cooking cavityinto which food is introduced and heated by microwave energy fed intothe cavity. In substantially all such ovens, a microwave tube of thetype known as a magnetron is employed as the microwave energy source.The magnetron in operation requires a source of high DC voltage and asource of filament or heater current, requirements which the magnetronhas in common with other types of electronic vacuum tubes. In practiceother conventional circuitry necessary for proper operation is providedin the power supply such as circuit breakers, On-Off switch, doorswitch, interlock switches, timers and switches, various time delaydevices and oven and indicator lamps.

There are several types of magnetrons available for these purposes andthese differ in detail from one another. For example, all magnetronshave an anode, a cathode, a filament and magnetic pole pieces and fluxsource. As is known, the filament and cathode are functionally'relatedin that it is the purpose of the filament to heat the cathode so thatthe latter efficiently emits elections. In one magnetron typeconstruction the filament is physically spaced from the cathode and thusindirectly heats the cathode. In another type of magnetron thefilament'and cathode are physically joined as a single unit and theheating is applied directly to the electron emitting substance of thecathode.

Obviously for a given current applied to the heater the warm-up time fora magnetron of the former construction is longer than the correspondingwarm-up time for the magnetron of the last named construction. In eithercase, however, the heater itself is a-thermally dependentresistance-being low in resistance when cold andbeing high in resistancewhen hot. Thus given a particular filament voltage, the initial currentpassing through the filament is higher than when the filament is heated.

In those prior art microwave ovens which incorporate a magnetron havingindirect heating of the cathode, the warm-up time of the magnetron wasunnecessarily long as a result of this indirectness of cathode heating.Hence circuit structure was incorporated in the power supply to increaseinitially the filament voltage above normal voltage levels. The purposeof this was to heat the cathode at a faster rate to reduce themagnetrons war 'm-up time. After the preheating period, the filamentvoltage was reduced to its normal level. In that type of magnetronconstruction, the heater operating temperature is on the order of I,400C. and maximum initial filament current or inrush" current, as may bevariously termed, is on the order of four times greater than the normalcurrent level. Increasing the filament voltage and currents dur ing thepreheat period was permissible and did not appear to have any harmfuleffects. In those microwave ovens which incorporate a magnetron of thetype containing a directly heated cathode such as the Litton L- 5001,however, the heater operating temperature is substantially higher,approximately l,760 C. Because the magnetron has a directly heatedcathode and operates at a higher temperature, the warm-up time of themagnetron is short and the preheat circuit previously discussed used inconnection with the indirectly heated cathodes in which the filamentvoltage is initially increased is neither necessary nor desirable,

Quite to the opposite, the higher operating temperatures of I,760 C.results in a heater in-rush current which is on the order of six timesgreater than the normal heater current level of 20 amps, reaching 150amps in a type L-SOOI. As a result of such extremely large currents intubes of this construction, a noticeable problem sometimes occurs in thedestruction of the heater and cathode.

Obviously there are many obvious ways by which this initial current canbe reduced. Thus conventional high leakage reactance transformers couldbe used to limit the in-rush current. Unfortunately, such types oftrans- Y formers for operation at low heater voltages are veryexpensive. Another obvious solution is to add a resistor in circuit withthe filament transformer in order to limit such current in-rush. Such aresistor, however, must be capable of dissipating at least 30 watts ofenergy and as such is expensive.

OBJECTS OF THE INVENTION BRIEF SUMMARY OF THE INVENTION In accordancewith the foregoing objects of my invention, a microwave oven includes anoven cavity, a magnetron for supplying microwave energy within the ovencavity, a power supply of the type which includes a filament transformerfor supplying filament current to the magnetron, a tungsten filamentlamp for illuminating the oven cavity. Control circuit means normallymaintain the primary winding of the filament transformer electrically inseries with the lamp across the l20-volt AC line resulting in a lowervoltage to each. In response to the initial opening of the oven cavitydoor the control circuit means connects each of the transformer primarywinding and the lamp in parallel across the l20-volt line so as to placeeach at full operating level. g

The foregoing and other objects and advantages of the invention,together with the elements and cooperation of the elementscharacteristic of my invention, together withsubstitute and equivalentelements therefor, becomes more apparent upon'consideration of thefollowing detailed description ofa preferred embodiment of the inventiontaken together with the figures of the drawings.

In the drawings:

FIG. 1 illustrates schematically the in-rush current limiting circuit ofthe invention in simplified form, and

FIG. 2 illustrates schematically a complete oven power supply circuitincluding the in-rush current limiting circuit of the invention.

DETAILED DESCRIPTION OF THE INVENTION The circuit of FIG. 1 includes: aplug 1 for connec tion to the 120-volt 60-cycle AC line conventionallysupplied by the electrical power company; a double pole single throwOn-Off switch 3; a filament transformer 5 containing primary winding 6and secondary winding 7, suitably for transforming 120 volts down to 5voltsacross secondary winding 7; a magnetron 9 which can suitably be aLitton type L-SOOl; an oven cavity 23; a tungsten filament lamp 25; ahigh voltage supply represented by the dashed lines 21; miscellaneousconventional timer and control circuits are symbolically represented bydashed lines 27; a normally closed shut-down switch 39; a door pulseswitch 33; and an electromagnetic relay 31. Relay 31 operates on 120-volt AC. The relay includes a set of normally open contacts, 32a and32b, a set of transfer contacts 34 which includes a break contact 34band a make contact 34m, and an additional set of transfer contacts 36which includes a break contact 36b and a make contact 36m. The line ,35is connected to terminal 3a of On-Off switch 3 and extends to one sideof primary winding 6. Lead 37 connects to the make contact 3b of theOn-Off switch and extends to one terminal of relay coil 31 to oneterminal of lamp 25 and to make contact 36m. The door pulse switch 33,normally open, is connected in circuit between line 35 and the otherterminal of relay coil 31. An additional connection is made between thatterminal of the relay and its make contact 32b as well as to the controlcircuits 27 via lead 41. The normally closed shut-down switch 39 isconnected in a series circuit between line 35 and the movable contact32a of relay 31 as well as to contact 34m of the transfer contacts 34.Another electrical circuit is completed between the remaining end of thetransformer primary winding 6 to transfer contact 36 and an electricalcircuit is completed between contacts 36b and 34b of the transfercontacts 36 and 34, respectively.

As indicated in the drawing, magnetron 9 includes heater terminals and17 which are connected across the secondary 7 of the filamenttransformer as well as to the negative -HV) high voltage terminal ofhigh voltage supply 21 via lead 19. An anode 11 of the magnetron isconnected via lead 13 to chassis ground potential. The positive polarityhigh voltage lead of supply 21 is likewise connected to chassis groundpotential. A suitable transmission line 18 completes a microwavetransmission circuit between magnetron 9 and oven cavity 23, and lamp asindicated is installed within or in proximity to the oven cavity so asto provide illumination within that cavity. Numerous fuses, circuitbreakers and interlock circuits, normally found as a practical matter inmicrowave ovens, are not included in this schematic inasmuch as they areconventional and not necessary to the understanding of the structure ofmy invention.

In normal operation plug 1, as illustrated in FIG. 1, is connectedacross the -volt 60-cycle AC line, and On-Off switch 3 is operated toits closed position. This applies the 120 volts across lines 35 and 37and completes a series circuit via line 35 to one side of primarywinding 6 through transfer contact 36, break con- I tact 36b, breakcontact 34b, transfer contact 34, lamp 25, to line 37.

Accordingly, current flows through primary winding 6 and lamp 25. Lamp25 provides illumination in oven cavity 23 at a slightly reduced level.Likewise the voltages induced in secondary winding 7 of the transformer5 are on the order of 1 volt which is considerably less than the normallevel of 5 volts. The filament voltage is applied to the heaterterminals 15 and 17 of magnetron 9 and heats the magnetron filament.

The oven remains in this condition until such time as the operatordesires to cook food. At that time, the oven cavity door is openedmomentarily as the food to be cooked is inserted into the oven cavityand the door is reclosed. This action momentarily operates door switch33 which momentarily closes a circuit between lines 35 and 37 to thecoil of relay 31.

Relay 31 operates and closes its contacts 32. This completes a holdingcircuit from line 35 through the normally closed switch 39, relaycontact 32, to the relay coil 31, which bypasses the circuit from line35 containing door switch 33, and relay coil 31 remains energized and inthe operated condition. Concurrently, relay transfer contacts 36 and 34switch from contact with the break contacts 36b and 3412, respectively,to the make contacts 36m and 34m, respectively. A circuit is completedfrom line 37 through lamp 25, transfer contact 34, make contact 34m,through normally closed switch 39 to line 35, and thus places the full120 volts across lines 35 and 37 across lamp 25. Accordingly, lamp 25provides full illumination to the oven cavity.

Another circuit is completed from the one side of primary winding 6through contact 36-make to line 37. The other side of primary winding 6is connected to line 35 as previously described. Thus primary winding 6is placed directly across the 120 volts appearing between line 35 and37. With the full 120 volts applied to primary winding 6, the filamentvoltage is then raised from the previous low level of 3 volts to thenormal level of 5 volts. Accordingly, a larger current flows to themagnetron filaments and the cathode of magnetron 9 is placed at itsnormal operating temperature.

It is noted that an additional circuit is completed at contacts 32 tothe miscellaneous circuits represented by the dashed lines 27 via line41. These include the timer circuits shutdown and miscellaneous controlcircuits which control the high voltage supply represented by the dashlines 21 which supplies high negative voltage via line 19 to themagnetron cathode which are necessary to the operation of a practicalmicrowave oven. These circuits are conventional and inasmuch as they arenot necessary or helpful to a clear illustration of the invention areomitted or simplified in this FIG. 1. In normal oven operation themiscellaneous circuits include a cook switch and timer circuit whichcloses the high voltage circuit and operates supply 21 which in turncauses magnetron 9 to generate microwave energy. This microwave energyis withdrawn from the magnetron by a transmission line 18 and passedinto the oven cavity.

Upon completion of the cooking operation the shutdown circuits, after anappropriate time delay, operates shutdown switch 39. Switch 39 opens andinterrupts the holding circuit to relay coil 31. Upon de-energization ofrelay coil 31, relay contacts 32 are returned to the normally openedposition, and relay contacts 34 and 36 are restored to their contactwith contacts 34b and 36b. Primary winding 6 and lamp are thus placedagain in a series circuit across lines 35 and 37.

Numerous modifications are apparent to the foregoing sequence ofoperation and to the elements of the invention. For example, by use of aconventional relay structure having make-before-break contacts, transfercontact 36 can be made of the make-before-break variety. In this way thecircuit to the primary of transformer 5 is completed or placed at thehigher voltage level prior to the time that the existing circuit isbroken. In so doing, a possible current surge through the primarywinding is eliminated and the effect of such a connection isto-momentarily darken lamp 25 which will flicker.

Additionally, the contacts can include the use of conventional arcsuppressing means across the contacts, typically capacitors orresistor-capacitor combinations. These are conventional and are used toprolong the life of relay contacts by preventing the damaging effects ofelectrical arcs that occur when a closed circuit is interrupted. Andwhile the switch used to connect the primary winding and lamp in seriesand in parallel'across the l20-volt line is shown to be anelectromechanical relay, it is apparent that available electronicswitches can be substituted for the relay if desired.

In one typical example, a 50-watt tungsten lamp was inserted in serieswith a primary winding supplying a L- 5001 magnetron. Due to the coldfilament of the lamp the first cycle (one-sixtieth second) in-rush was100 amps peak and dropped to amps peak for the second cycle and finallystabilized at 13 amps peak. When the filament transformer primary wasplaced directly across the line, the filament current rose to 60 ampspeak initially and quickly settled to the equilibrium value of 27 ampspeak. In this combination, therefore, the destructive consequences ofin-rush currents of one-half second or so duration and 70 to 150 ampspeak have been avoided. Obviously, the initial current can be variedsomewhat by varying the lamp wattage size and hence vary the proportionof voltage across the transformer primary winding.

Although the foregoing description presented in connection with theembodiment of FIG. 1 is adequate and concisely and clearly illustratesthe invention, it is perhaps even more informative to illustrate anddescribe the invention in connection with the complete and detailedcircuit of a commercial microwave oven as I present in FIG. 2.

For convenience and simplicity of illustration, the schematic of FIG. 2is drafted using the principles ofdetached contact" form of drawings.Briefly, in the detached contact form of schematic drawing the relaycoil is located at any convenient location and the relay contacts in theschematic are separated physically and placed where convenient incontrast to the conventional form of drawing in which a relay coil andall ofits associated contacts are located together. Additionally in thisform of electrical drafting, relay contacts are represented by Xs anddashes. More particularly when a pair of relay contacts are normallyopen and are to make, that is close those contacts, upon energization ofthe relay coil, such pair of contacts are presented as an X in thedrawings. Likewise, if a pair of relay contacts are normally closed andinstead are opened upon energization of the relay coil to break orinterrupt a circuit path, this form of contact is represented in theschematic as a dash drawn through a circuit line. Combining these twofeatures, a set of relay transfer contacts which conventionally includesa transfer contact which normally abuts a break" contact, and which,upon energization of the relay coil, moves from contact with the breakcontact into contact with the third or make contact, is represented byboth an X and by a dash adjacent one another, and the juncture betweenthe X and dash symbols represents the location of the movable transfercontact of the relay.

Given a relay such as K1 in FIG. 2 by this convention theelectromagnetic coil of the relay is represented by K1 and the termsrelay and relay coil are synonymous. The first set of relay contacts onthe relay is a transfer contact and this is denoted in the drawing asKl-l which identifies the transfer contacts as the first set of contactson relay Kl. The make contact of this set can be referred to as contactKl-lm and the break as KI-lb. Likewise additional sets of contacts areillustrated such Kl-2, another set of transfer contacts, and K1-3, a setof make contacts. For regular switches,

. the conventional symbols may be used.

With the foregoing drafting conventions in mind, the description of thecomplete power supply arrangement detailed and illustrated in FIG. 2 maybe clearly understood. The power supply is connected to a source ofl20-volt 60-cycle AC line current by means of a suitable plug, P, toapply the line voltage across lines 51 and 53. Line 51 is connected toone contact of On-Off switch S1, a double pole single throw switch,contact 81-1 in series with a second timer interlock switch, S2, and acircuit breaker, CB, to a point or terminal designated A. Line 53 isconnected in series with contacts 81-2 of switch S1 to a point orterminal B. Switch S1 is the On-Off switch for the oven power supply andfor safety includes contacts which interrupt or open both sides of thehousehold AC line in the Off position. The terminals A and B are used inthe drawings as a reference point and, hence, where other elements areconnected to A or B, it is understood that they are in fact wiredelectrically in common with those reference points.

The transfer contact, S3, the oven door switch is connected to one sideof relay coil K1 to the make contact of relay hold contact K1-3 and tofuse F. The make contact of switch S3 is wired to terminal A. The otherterminal of relay coil K1 is connected in series through break contactTDl-l to terminal B. The other contact of relay make contact K1-3 isconnected to terminal A. Fuse F is connected to one side of circuitbreaker TPl, lamps PL2 and PL3, blower motor M1, and time delay heatercoil TD2. The other side of heater coil TD2 and blower motor Ml areconnected to terminal B. Lamp PL2 is connected to the break contact andlamp PL3 is connected to the make contact, respectively, of relaytransfer contacts K3-2 and the transfer contact of K3-2 is connected toterminal B.

The other terminal of circuit breaker TPl is connected to one terminalof relay coil K2, relay coil K3 and timer motor M3. The other terminalof relay coil K2 is connected through a make contact TD2 of delay relayTD2 through break contacts 81-3 to the other terminal of relay coil K3.A parallel circuit is wired from that terminal of relay coil K3 throughits make contacts K3-l and through the normally open cook switchcontacts S6 and from there through normally open make contacts M3 of thetimer mechanism to the other terminal of the timer motor.

The remaining terminal of timer motor M3 is wired in parallel with theremaining break contact of contacts M3-l in series with normally closedtimer home switch Th to point B of the power line. The timer is of aconventional structure and provides the means to select the cookingtime. At the end of the selected timed interval the timer operates itscontacts Th to open the circuit.

An additional circuit is provided between an interlock contact on doorswitch S3 to the transfer contact of contacts K2-1 of relay K2. Thebreak contact of K2- 1 is connected to one side of the heater coil oftime delay relay TDl. The other side of the heater coil of TD] isconnected through resistor R1 to terminal B of the power lines. The makecontact of K2-l is connected in parallel to one side of stirrer motor M2and to one terminal of the primary winding of a transformer T2, the highvoltage transformer. The other terminal of both stirrer motor ML andprimary winding of transformer T2 isconnected to terminal B. Thesecondary winding of the transformer T2 is connected in series withcapacitor CF to a diode bridge rectifier consisting of diodes D1, D2, D3and D4. The positive output terminal of the bridge rectifier isconnected to chassis ground and the negative output terminal isconnected to the secondary low voltage filament winding of filamenttransformer T1. The make contact of transfer contacts Kl-2 is connectedto terminal A; the break contact of Kl-2 is connected to the breakcontact K1- 1; and the transfer contact of K1-2 is connected to one sideof the primary winding of stepdown filament transformer Tl.

A second circuit from terminal A is made through lamp LA to the transfercontact of relay contact Kl-l. The make contact of contact Kl-l isconnected to terminal B. The remaining terminal of transformer T1primary winding is also wired to terminal B.

The secondary winding of transformer T1 is connected through an L-Cfilter located in a shielded enclosure 55 to the heater terminals of amagnetron 57. The magnetron anode is connected to chassis ground and theoutput terminal of the magnetron is connected through suitabletransmission lines 59 to the oven cavity, not illustrated.

In operation with plug P installed in the electrical path outlet theOn-Off switch S1 is operated to its On position which closes contacts81-1 and 51-2. Assuming normal operation, switch S2 and circuit breakerCB are also closed and hence this circuit provides volts AC across theterminals A and B. Reference to the circuit containing lamp LA is made.A current path exists from terminal A through lamp LA, break contactKl-l, break contact Kl-2, the primary winding of transformer T1 toterminal B. In this way, a reduced voltage is applied across the primarywinding of transformer T1 and this is reflected in a lower outputvoltage being applied to the heaters of magnetron 7. Lamp LA operates atreduced intensity. Any current surge through this circuit are obviouslyminimized by the resistance of lamp LA.

By means of the foregoing circuit path, a reduced voltage is applied tothe primary winding of filament transformer T1 which'appears as alower-than-normal filament voltage a the secondary of the filamenttransformer. Accordingly, the magnetron heater to which the filamenttransformer secondary is coupled is'preheated.

At any time subsequent to operation of the On-Off switch 81-1, theoperator will desire to commence cooking or heating of food. This can beeither seconds, minutes, or even hours later and, as becomes apparentfrom the following description, the magnetron heater is preheated asdescribed for essentially the same period of time.

At the time the operator desires to cook food, the oven door is open,the food inserted, and the door is reclosed. This results in momentaryoperation of door switch S3. Accordingly, a momentary path existsbetween terminal A, make contact of door switch S3, relay coil Kl,through closed contacts TDl-l to terminal B, and relay coil K1 isenergized. Relay K1 closes its make contacts Kl-3 which completes analternative current path from terminal A to relay coil K1, and hencerelay K1 is held in its operated condition regardless of the subsequentopening of door switch S3. Relay contacts Kl-2 and Kl-l are alsooperated. Transfer contact of contacts K1-2 interrupts the break circuitand completes a circuit via the make contact between terminal B, lampLA, and terminal A, which results in full energization of the lamp andfull intensity illumination of the oven cavity. The transfer contact ofK1-2 interrupts the break circuit and completes a circuit via the makecontact to terminal A, resulting in a current path between terminal A,transformer T1 primary winding and terminal B, and thus results in theapplication of full line voltage to the filament transformer primarywinding. In so doing, the voltage at the secondary winding of T1 israised to normal heater voltage.

At this time, additional current paths are completed from point A,contact Kl-3, fuse F to pilot lamp PL2, to blower motor M1, and tothermal time delay relay TD2, as well as to one side of relays K2 and K3and timer motor M3. At this time, lamp PL3 is operated to indicate thatthe oven is in a ready condition and blower motor M1 commences operationof the conventional blower which channels cooling air to the magnetron.Timer motor M3 is energized and closes contacts M3. The timerillustrated is set to the desired time and upon depression of cookerswitch S6 a circuit is completed through relay coil K3, switch S6,contacts M3 to terminal B. Relay K3 closes a make circuit at contactK3-l to provide an alternative holding current path to terminal B inorder to maintain relay K3 in the operated condition. Upon operation,contacts K3-2 transfers from the break to the make condition, thusclosing a current circuit to lamp PL3 and extinguishing lamp PL2 whichthus indicates other food inserted into the cavity is cooking.Concurrently with this operation, TD2, after a time delay ofapproximately seconds, closes contacts TD2-1 in the circuit of relay K2.Relay K2 operates and through contacts of K3-1 is held energized.Contacts K2-1 close a make circuit from terminal A, contacts K1-3,switch S3, to stirrer motor M2 and primary winding of transformer T2 toterminal B.

The oven stirrer, not illustrated, operates and the l-volt line voltageis transformed by transformer T2 to the high voltages, typically 2,500volts, which appear across the secondary winding. This voltage isrectified by the bridge rectifier consisting of diodes D1 through D4 andapplied to the heater lead of magnetron 7. Since the cathode isconnected to the heater within the tube the negative voltage is appliedto the cathode. Both the anode of magnetron 57 and the other end ofbridge rectifier are connected electrically to chassis ground and thuscomplete a high voltage circuit.

In normal operation the magnetron delivers its microwave energy viatransmission line 59 to the oven cavity and results in cooking of thefood.

As thus described it is evident how the invention illustrated in FIG. 1and as appears in detail in H0. 2 cooperates with the other conventionalelements of a microwave oven power supply circuit to preheat themagnetron filament and limit in-rush current. However for convenience,the further sequence of events is also helpful. Upon completion of thecooking cycle as determined by the timer, contact Th4 is opened andresults in the de-energization of relay K3. in de-energizing, relay K3opens the circuit to relay K2 which also de-energizes, closing a circuitvia contacts K2-l, break to time delay TD] and opening the highvoltagecircuit to transformer T2. Should further cooking operation follow,relays K2 and K3, cooking switch S6 and timer motor M3 will be operatedagain in the same manner as previously described, without further changein the energized state of relays 'Kl, time delay TD2 and blower motorMl, which remain on for a predetermined time. lf no further cookingcycle occurs after perhaps 90 seconds, time delay TDl operates and openscontacts TDl-l to break the circuit to relay Kl. Relay K1 is thende-energized and restores to normal contacts K1-3, contacts K1-2, andcontacts Kl-l,

restoring the series circuit of lamp LA and transformer primary windingT1. Blower motor M1 is also de-energized and the entire circuit remainsin that condition until door switch S3 is subsequently closed at thecommencement of another cooking cycle.

The lamp preferably should have a voltage rating equal to that of theline voltage to be supplied to the oven at the plug and a power ratingin watts within a range of 30 to 65 percent.'for example 50 percent, ofthat power normally consumed by the magnetron heater. In a specificexample, the line voltage in the embodiments of FIGS. 1 and 2 is 120volts and the magnetron heater in normal operation outside any inrushperiod is supplied approximately 5 volts at 20 amperes or, in terms ofpower, lOO watts. In this specific example therefore the lamp is ratedat 120 volts and 40 WflttS.

It is apparent to those skilled in the art that numerous modificationscan be made to the disclosed preferred embodiments of my inventioneither to change the details of the elements thereof or to substituteobvious equivalents therefor as well as to change slightly the sequenceof operations as a result of differences in power supply circuits andthus vary the functional relationship of the invention with that of theother elements comprising the power supply. It is thus expresslyunderstood that such details have been presented in this specificationonly as illustrative of my invention so as to teach one skilled in theart how to make and use my invention and not by way of limitation, andthat all such changes, modifications, and substitutions or improvementsfall within the scope of my invention. Accordingly, it is requested thatmy invention be broadly construed, limited only by the breadth and scopeof the appended claims.

What is claimed is:

1. In a microwave oven which includes:

an oven cavity for receiving food to be cooked;

a door for providing access to said oven cavity;

a magnetron for supplying microwave energy into said oven cavity;

lamp means for illuminating said oven cavity with light;

power supply means for said magnetron, said power supply means includinga filament transformer having a primary winding and a low voltagesecondary winding and circuit means connecting said secondary winding tosaid magnetron filaments; and

power lines for providing power to said power supply means;

the invention comprising in combination therewith; first circuit meansfor placing said primary winding and said lamp means electrically in aseries circuit across said power lines, and switch means responsive tooperation of said cavity door for interrupting said series circuit andplacing each of said primary winding and said lamp means individually inparallel across said power lines.

2. The invention as defined in claim 1 wherein said lamp means comprisesa light bulb having a tungsten filament.

3. The invention as defined in claim 1 wherein said switch meanscomprises an electromechanical relay, said relay including contact meansfor interrupting said series circuit of primary winding and lamp meansand having additional contact means for connecting each of said primarywinding and lamp means individually across said power lines.

4. The invention as defined in claim 3 further comprising an On-Offswitch for completing a circuit between said oven power supply means andexternal power lines for initially preparing said series current paththrough said lamp means and said primary winding.

5. In a microwave oven which includes:

an oven cavity for receiving food to be cooked;

lamp means for illuminating said oven cavity, said lamp means having anelectrically resistive filament;

a magnetron for supplying microwave energy into said oven cavity, saidmagnetron having a directly heated cathode construction and a filament;

magnetron power supply means, said magnetron power supply meansincluding a filament transformer having a primary winding and asecondary winding and circuit means connecting said secondary winding tosaid magnetron filaments;

power line means for supplying power at a predetermined voltage to saidmagnetron power supply means;

the invention comprising in combination therewith: first circuit meansfor placing said primary winding and said lamp means electrically in aseries circuit across said power lines and switch means responsive tooperation of said cavity door for interrupting said series circuit andplacing each of said primary winding and said lamp means in parallelacross said power lines. 4

6. A microwave oven including:

an oven cavity;

a movable door for permitting access to said cavity;

lamp means for illuminating said cavity;

said lamp means having a tungsten filament;

a magnetron for generating microwave energy for transmission into saidoven cavity, said magnetron containing a directly heated cathodestructure and heater terminals;

power supply means for said magnetron, said power supply means includinga filament transformer, said filament transformer having a primarywinding containing first and second terminals and a low voltagesecondary winding;

means connecting said secondary winding of said transformer to saidheater terminals of said magnetron;

power circuit means for supplying line voltage to said power supplymeans, said circuit means applying said line voltage across first andsecond terminals;

first On-Off switch means in series circuit with said power circuitmeans for completing a circuit to said first and second terminals;

electromagnetic relay means, said relay means including a first set ofnormally open contacts, a second set of transfer contacts, and a thirdset of transfer contacts, each of said sets of transfer contactsincluding a break contact, a make contact and a transfer contact, saidtransfer contact normally maintains closed a circuit to said breakcontact and which upon energization of said relay interrupts said closedcircuit and closes a normally open circuit to said make contact;

' door switch means connected in a series circuit, said relay meansbetween said first and second terminals for energizing said relay meansupon operation of said oven door;

means connecting together in circuit, said break contacts of each ofsaid first and second sets of relay contacts;

means connecting said primary winding in circuit between said transfercontact of said first set and said first terminal of said power circuitmeans;

means connecting said lamp means in circuit between said transfercontact of said second set and said second terminal of said powercircuit means to form a series circuit with said primary winding andsaid lamp means;

and means connecting said make contact of each of said second and firstsets of relay contacts in circuit with said first terminal and saidsecond terminal, respectively, of said ower circuit means; and meansconnecting said irst normally open contacts in circuit with said relaymeans and said power circuit means for maintaining said relay meansenergized independent of said door switch, subsequent to initialoperation of said door switch; whereby upon closure of said On-Offswitch current flows through said primary windings and said lamp meansto provide reduced low voltage to said magnetron filaments andilluminate said oven cavity and whereby upon initial operation of saidoven door said relay means operates and is maintained energized andplaces each of said lamp means and said primary winding individuallyacross said power lines to fully illuminate said oven cavity and providefull heater voltage to said magnetron.

7. The invention as defined in claim 6 wherein said magnetron heater israted to consume normally a predetermined X watts of energy and whereinsaid line voltage from said power circuit means is a predetermined Yvolts, and wherein said lamp means has a voltage rating of Y volts and anominal power consumption rating within the range of 30 percent to 65percent of said X watts.

8. The invention as defined in claim 5 wherein said magnetron filamentis rated at a nominal predetermined power consumption of X watts andwherein said power line means supplies a voltage of Y volts and whereinsaid lamp means has a voltage rating of Y volts and a power consumptionrating in the range of 30 percent to 65 percent of said X watts.

1. In a microwave oven which includes: an oven cavity for receiving foodto be cooked; a door for providing access to said oven cavity; amagnetron for supplying microwave energy into said oven cavity; lampmeans for illuminating said oven cavity with light; power supply meansfor said magnetron, said power supply means including a filamenttransformer having a primary winding and a low voltage secondary windingand circuit means connecting said secondary winding to said magnetronfilaments; and power lines for providing power to said power supplymeans; the invention comprising in combination therewith; first circuitmeans for placing said primary winding and said lamp means electricallyin a series circuit across said power lines, and switch means responsiveto operation of said cavity door for interrupting said series circuitand placing each of said primary winding and said lamp meansindividually in parallel across said power lines.
 2. The invention asdefined in claim 1 wherein said lamp means comprises a light bulb havinga tungsten filament.
 3. The invention as defined in claim 1 wherein saidswitch means comprises an electromechanical relay, said relay includingcontact means for interrupting said series circuit of primary windingand lamp means and having additional contact means for connecting eachof said primary winding and lamp means individually across said powerlines.
 4. The invention as defined in claim 3 further comprising anOn-Off switch for completing a circuit between said oven power supplymeans and external power lines for initially preparing said seriescurrent path through said lamp means and said primary winding.
 5. In amicrowave oven which includes: an oven cavity for receiving food to becooked; lamp means for illuminating said oven cavity, said lamp meanshaving an electrically resistive filament; a magnetron for supplyingmicrowave energy into said oven cavity, said magnetron having a directlyheated cathode construction and a filament; magnetron power supplymeans, said magnetron power supply means including a filamenttransformer having a primary winding and a secondary winding and circuitmeans connecting said secondary winding to said magnetron filaments;power line means for supplying power at a predetermined voltage to saidmagnetron power supply means; the invention comprising in combinationtherewith: first circuit means for placing said primary winding and saidlamp means electrically in a series circuit across said power lines andswitch means responsive to operation of said cavity door forinterrupting said series circuit and placing each of said primarywinding and said lamp means in parallel across said power lines.
 6. Amicrowave oven including: an oven cAvity; a movable door for permittingaccess to said cavity; lamp means for illuminating said cavity; saidlamp means having a tungsten filament; a magnetron for generatingmicrowave energy for transmission into said oven cavity, said magnetroncontaining a directly heated cathode structure and heater terminals;power supply means for said magnetron, said power supply means includinga filament transformer, said filament transformer having a primarywinding containing first and second terminals and a low voltagesecondary winding; means connecting said secondary winding of saidtransformer to said heater terminals of said magnetron; power circuitmeans for supplying line voltage to said power supply means, saidcircuit means applying said line voltage across first and secondterminals; first On-Off switch means in series circuit with said powercircuit means for completing a circuit to said first and secondterminals; electromagnetic relay means, said relay means including afirst set of normally open contacts, a second set of transfer contacts,and a third set of transfer contacts, each of said sets of transfercontacts including a break contact, a make contact and a transfercontact, said transfer contact normally maintains closed a circuit tosaid break contact and which upon energization of said relay interruptssaid closed circuit and closes a normally open circuit to said makecontact; door switch means connected in a series circuit, said relaymeans between said first and second terminals for energizing said relaymeans upon operation of said oven door; means connecting together incircuit, said break contacts of each of said first and second sets ofrelay contacts; means connecting said primary winding in circuit betweensaid transfer contact of said first set and said first terminal of saidpower circuit means; means connecting said lamp means in circuit betweensaid transfer contact of said second set and said second terminal ofsaid power circuit means to form a series circuit with said primarywinding and said lamp means; and means connecting said make contact ofeach of said second and first sets of relay contacts in circuit withsaid first terminal and said second terminal, respectively, of saidpower circuit means; and means connecting said first normally opencontacts in circuit with said relay means and said power circuit meansfor maintaining said relay means energized independent of said doorswitch, subsequent to initial operation of said door switch; wherebyupon closure of said On-Off switch current flows through said primarywindings and said lamp means to provide reduced low voltage to saidmagnetron filaments and illuminate said oven cavity and whereby uponinitial operation of said oven door said relay means operates and ismaintained energized and places each of said lamp means and said primarywinding individually across said power lines to fully illuminate saidoven cavity and provide full heater voltage to said magnetron.
 7. Theinvention as defined in claim 6 wherein said magnetron heater is ratedto consume normally a predetermined X watts of energy and wherein saidline voltage from said power circuit means is a predetermined Y volts,and wherein said lamp means has a voltage rating of Y volts and anominal power consumption rating within the range of 30 percent to 65percent of said X watts.
 8. The invention as defined in claim 5 whereinsaid magnetron filament is rated at a nominal predetermined powerconsumption of X watts and wherein said power line means supplies avoltage of Y volts and wherein said lamp means has a voltage rating of Yvolts and a power consumption rating in the range of 30 percent to 65percent of said X watts.